The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with single-photon emission computed tomography (SPECT) with single or multi-headed cameras and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in other nuclear medicine and transmission radiation diagnostic imagers.
Heretofore, single photon emission computed tomography has been used to study a radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals are injected into a subject. The radiopharmaceuticals are commonly injected into the subject's blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera heads are placed closely adjacent to a surface of the subject to monitor and record emitted radiation. In single photon-emission computed tomography, the head is rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions is reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the subject.
A drawback to the SPECT imaging technique is that the patient is not completely homogeneous in terms of radiation attenuation or scatter. Rather, the human patient includes many different tissue and bone types which absorb or scatter radiation from the radiopharmaceuticals to different degrees. The SPECT images can be made more accurate if they are corrected for the radiation lost to scattering or attenuation along each path through the human torso.
Accordingly, transmission radiation sources have been placed opposite the patient from a detector head. In three detector head systems, for example as disclosed in U.S. Pat. No. 5,479,021, which is commonly owned with the present application, the fan beam radiation source is mounted to the rotating gantry between two of the detectors and opposite the third. Such a mounting arrangement is of course not applicable to opposed, two detector head systems.
In single head systems, for example as disclosed in Tan, A Scanning Line Source for Simultaneous Emission and Transmission Measurements in SPECT, J. Nuclear Med., Vol. 34, No. 10, Pg. 1752 (October 1993) a scanning line source is mounted on a frame attached to the collimator of the single head. This technique is also inapplicable to opposed two headed systems, particularly in light of the of the line source's height, the need to provide effective shielding between the line source and the second detector while reducing the effective height of the source assembly, the limitations imposed by the frame, and the inability to adjust the relative distances between the transmission source, the object being imaged, and the detector.
Scanning line sources have also been used in two head right angle systems wherein the detectors are mounted at a 90.degree. angle to each other. In such a system, however, close body orbits are problematic because one head can get in the way of the other and the patient is not centered in the field of view. Accordingly, it is desirable to apply a line source in a system having opposed detectors.
One technique for transmission imaging in an opposed detector system is to mount the line source at the side of one of the opposed detectors. A significant drawback to this approach is that the collimator of the opposed detector must be modified to allow detection of the transmitted radiation, which can increase patient to detector distance and adversely affect resolution and image quality. Such a modification has a deleterious effect on the detector's field of view.
Yet another drawback to the many prior art line source techniques is that radiation emitted by the line source but not attenuated by the subject reaches the detector without substantial attenuation. This "shine by" radiation results in extraneous detector counts and can cause saturation of the detector, leading to inaccuracies in the image data. One attempt to reduce the "shine by" radiation is disclosed in U.S. Pat. No. 5,576,545 ('545) assigned to Siemens Medical Systems, Inc. The '545 patent discloses the use of five mechanically rotatable shutters which can each be selectively positioned to attenuate radiation emitted by the line source. Accordingly the '545 patent discloses attenuating a greater percentage of the radiation as the line source is moved past regions where radiation would have otherwise "shined by" an object being imaged. This reduces the possibility of saturating the detector. In the '545 patent, the amount of attenuation occurring at any given time is governed by the number of shutters positioned in front of the line source. By rotating more shutters in front of the line source, or in other words by varying the overall thickness of attenuating material which the radiation from the line source must pass, the amount of attenuation is varied. Unfortunately there are several drawbacks to the solution proposed in the '545 patent. One drawback is that the proposed solution requires many mechanical and electrical components to control the positioning of the shutters adding cost and complexity to the system. Further, the '545 solution only provides for attenuation of the line source radiation in a direction perpendicular to a longitudinal axis of the examination region thereby limiting its usefulness.
The present invention contemplates a new and improved scanning line source which is particularly suited to two headed gamma cameras and other gamma cameras having opposed detector heads. The present invention further contemplates a technique for shaping the intensity of the transmitted radiation so as to reduce the undesirable effects of shine by radiation. As described more fully below, present invention overcomes the above-referenced problems and others.